The present invention relates generally to ceramic matrix composite structures, and more particularly to a ceramic matrix composite component having integral cooling passages formed therein.
Combustion turbines are well known in the art as having a compressor section for supplying a flow of compressed combustion air, a combustor section for burning a fuel in the compressed combustion air, and a turbine section for extracting thermal energy from the combustion air and converting that energy into mechanical energy in the form of a shaft rotation. Many parts of the combustor section and turbine section are exposed directly to the hot combustion gasses, for example the combustor, the transition duct between the combustor and the turbine section, and the turbine stationary vanes, rotating blades and surrounding ring segments.
It is also known that the power and efficiency of a combustion turbine may be increased by increasing the firing temperature of the combustion gas. Modern, high efficiency combustion turbines may have firing temperatures in excess of 1,600xc2x0 C., which is well in excess of the safe operating temperature of the structural materials used to fabricate the hot gas flow path components. Accordingly, several methods have been developed to provide cooling for such components, including film cooling, back-side cooling and insulation.
Film cooling involves the delivery of a film of cooling fluid, such as compressed air extracted from the compressor section, between the structural component and the flow of hot combustion gasses. The film of cooling fluid may be provided through holes formed in the surface of the component which are in fluid communication with the compressor section. Film cooling systems are generally very effective in cooling a component, however they may significantly reduce the efficiency of the machine. Energy is needed to compress the cooling fluid, a decrease in combustion gas temperature is induced by the addition of the relatively cold fluid, and disturbance may be created in the smooth flow of air over an airfoil component such as a blade or vane.
Back-side cooling generally involves the passage of a cooling fluid over a back side of a component that has a front side exposed to the hot combustion gasses. The cooling fluid in back-side cooling schemes may be compressed air that has been extracted from the compressor or steam that is available from other fluid loops in a combustion turbine power plant. Back-side cooling does not affect the exhaust gas composition or the flow of air over an airfoil component, it does not dilute the hot combustion air with colder fluid, and it can generally be supplied at a lower pressure than would be needed for film cooling. However, back-side cooling creates a temperature gradient across the thickness of the cooled wall, and thus becomes decreasingly effective as the thickness of the component wall increases and as the thermal conductivity of the material decreases.
Finally, insulation materials, such as ceramic thermal barrier coatings (TBC""s), have been developed for protecting temperature-limited components. While TBC""s are generally effective in affording protection for the current generation of combustion turbine machines, they may be limited in their ability to protect underlying metal components as the required firing temperatures for next-generation turbines continue to rise.
Ceramic matrix composite (CMC) materials offer the potential for higher operating temperatures than do metal alloy materials due to the inherent nature of ceramic materials. This capability may be translated into a reduced cooling requirement which, in turn, may result in higher power, greater efficiency, and/or reduced emissions from the machine. However, CMC materials generally are not as strong as metal, and therefore the required cross-section for a particular application may be relatively thick. Due to the low coefficient of thermal conductivity of CMC materials and the relatively thick cross-section necessary for many applications, back side closed loop cooling is generally ineffective as a cooling technique for protecting these materials in combustion turbine applications. Accordingly, a high temperature insulation for ceramic matrix composites has been described in U.S. Pat. No. 6,197,424 B1, which issued on Mar. 6, 2001, and is commonly assigned with the present invention. That patent describes an oxide-based insulation system for a ceramic matrix composite substrate that is dimensionally and chemically stable at a temperature of approximately 1600xc2x0 C. However, even higher operating temperatures are envisioned for future generations of combustion turbine machines. Accordingly, an improved method of cooling a ceramic matrix composite material is needed. Furthermore, a ceramic matrix composite material capable of operating at temperatures in excess of 1600xc2x0 C. is needed.
A multi-layer ceramic matrix composite structure is disclosed herein to have a top layer of ceramic matrix composite material; a bottom layer of ceramic matrix composite material; and an intermediate layer of ceramic matrix composite material joining the top layer and the bottom layer, the intermediate layer including a plurality of adjoining hollow ceramic matrix composite structures, each hollow ceramic matrix composite structure in integral contact with the top layer, the bottom layer and respective adjoining hollow ceramic matrix composite structures, the hollow ceramic matrix composite structures defining a respective plurality of cooling passages through the multi-layer ceramic matrix composite structure. Reinforcing fibers in the hollow ceramic matrix composite structures may be oriented circumferentially, longitudinally or in a spiral configuration, thereby providing additional strength to the structure in the region surrounding the cooling passages.
A method of fabrication is described herein as including the steps of: providing a bottom layer of ceramic fiber material; wrapping ceramic fiber material around a fugitive material to form a plurality of ceramic fiber wrapped fugitive material structures; disposing the plurality of ceramic fiber wrapped fugitive material structures on the bottom layer; disposing a top layer of ceramic fiber material over the plurality of ceramic fiber wrapped fugitive material structures to form a layered structure; impregnating the layered structure with a ceramic matrix precursor material; and applying a compressive force and heat to the impregnated structure to eliminate voids in the impregnated structure by deforming the fugitive material structures and to dry and cure the matrix precursor material to form a green body structure. Further steps may include heating the green body structure to a temperature sufficiently high to remove the fugitive material to form a plurality of cooling passages.
In a further embodiment, a multi-layer ceramic matrix composite structure is described as including: a top layer of ceramic matrix composite material; a bottom layer of ceramic matrix composite material; a plurality of hollow ceramic matrix composite structures; and an intermediate layer of ceramic matrix composite material disposed between the top layer and the bottom layer; the intermediate layer having a generally serpentine cross-sectional shape disposed alternately over and under adjacent ones of the plurality of hollow ceramic matrix composite structures.
In a further embodiment, a multi-layer ceramic matrix composite structure is described as including: a top layer of ceramic matrix composite material; a bottom layer of ceramic matrix composite material; a plurality of hollow ceramic matrix composite structures; and an intermediate layer of ceramic matrix composite material disposed between the top layer and the bottom layer; the intermediate layer having a generally serpentine cross-sectional shape disposed alternately over and under adjacent ones of the plurality of hollow ceramic matrix composite structures.
A further method of fabricating is described as including: providing a plurality of pins formed of a fugitive material; weaving a mat of ceramic fibers around the plurality of pins of fugitive material; impregnating the mat with a matrix precursor material; drying and curing the matrix precursor material; and removing the fugitive material to form a plurality of passages through the mat. The method may further include wrapping each of the plurality of pins with ceramic fibers prior to the step of weaving.
Another embodiment is further described as including: a multi-layer ceramic matrix composite material; a layer of ceramic thermal barrier coating material disposed on the multi-layer ceramic matrix composite material; and a cooling passage formed in the multi-layer ceramic matrix composite material, the cooling passage having a longitudinal axis extending in a direction generally parallel to a plane of a layer of the multi-layer ceramic matrix composite material, the cooling passage bounded by a layer of ceramic matrix composite material having fibers disposed around the longitudinal axis.